1. Field of the Invention
The present invention is a fuel pressure control apparatus for a multicylinder internal combustion engine which is capable of injecting fuel into respective cylinders of the multicylinder internal combustion engine while controlling the pressure of fuel in a fuel rail at a high pressure. More particularly, the invention relates to such a fuel pressure control apparatus for a multicylinder internal combustion engine that is provided with a high pressure fuel pump having N (N<M) times of fuel discharge stroke with respect to the fuel rail during one round of the fuel injection stroke to M pieces of cylinders.
2. Description of the Related Art
In recent years, there has been put into practical use internal combustion engines in which the pressure of fuel in a fuel rail is controlled to a desired target high pressure value so as to inject the fuel in an atomized state (see, for example, a first patent document (Japanese patent No. 2890898) and a second patent document (Japanese patent application laid-open No. 11-324757)). Hereinafter, an example of the construction of a fuel system in such a kind of four-cylinder internal combustion engine will be described. A high pressure fuel pump for pressurizing fuel to a high pressure is provided with a plunger that reciprocates in a cylinder defining therein a pressure chamber, with a lower end of the plunger being made into pressure contact with a cam mounted on a camshaft of the internal combustion engine. With this arrangement, as the cam rotates in conjunction with the rotation of the camshaft, the plunger is caused to reciprocate in the cylinder, whereby the volume of the pressure chamber is changed.
Here, note that the plunger is reciprocated in the cylinder three times during one revolution of the camshaft, and hence in case of the four-cylinder internal combustion engine, it performs three reciprocations during the time when the fuel injection stroke to the respective cylinders of the engine makes a round (i.e., during two revolutions of the internal combustion engine). In addition, an inflow passage upstream of the pressure chamber is connected with a fuel tank through a check valve, a low pressure pump and a low pressure regulator, so that when the plunger is moved downward in the pump cylinder, the fuel discharged from the low pressure pump, after being adjusted to a predetermined low pressure by means of the low pressure regulator, is introduced into the pressure chamber through the check valve.
On the other hand, a feed passage downstream of the pressure chamber is connected with a fuel rail through a check valve, so that the fuel rail serves to hold the high pressure fuel discharged from the pressure chamber and distribute it to fuel injection valves. Here, note that the check valve in the feed passage serves to check or restrict the backflow of fuel from the fuel rail to the pressure chamber. In the four-cylinder internal combustion engine, a total of four of fuel injection valves are provided one for each engine cylinder.
In addition, a normally closed relief valve, being opened at a predetermined valve opening pressure or above, is connected with the fuel rail, and when the fuel pressure in the fuel rail is about to rise to or above the predetermined valve opening pressure value set for the relief valve, the relief valve is opened to return the fuel in the fuel rail to the fuel tank through the relief passage, thereby preventing an excessive increase in the fuel pressure.
A fuel discharge amount control valve in the form of a normally open electromagnetic valve for example is arranged between the feed passage and a spill passage, so that when the plunger of the high pressure fuel pump is caused to move upward in the pump cylinder, fuel discharged from the pressure chamber to the feed passage is returned from the spill passage to the inflow passage during the time the fuel discharge amount control valve is controlled to open, as a result of which high pressure fuel is not supplied to the fuel rail. After the fuel discharge amount control valve is closed at a predetermined timing during the upward movement of the plunger in the pump cylinder, the pressurized fuel discharged from the pressure chamber to the feed passage is supplied to the fuel rail through the check valve.
A control unit in the form of an ECU (electronic control unit) determines a target fuel pressure based on the operating condition of the internal combustion engine, and controls the drive timing of the fuel discharge amount control valve so that the fuel pressure in the fuel rail is made to coincide with the target fuel pressure. In addition, the ECU also specifies the rotational angle position of the internal combustion engine based on the rotational phase of a crankshaft and the rotational phase of a camshaft, and calculates the amount of fuel per cylinder to be injected to each engine cylinder based on the amount of depression of an accelerator pedal, whereby the fuel injection valves are driven to operate under the control of the ECU.
Next, reference will be made to the operation of controlling the amount of fuel to be discharged. In the fuel suction stroke in which the plunger of the high pressure fuel pump is caused to move downward from the upper end up to the lower end of the pump cylinder, low pressure fuel is sucked from the suction passage into the pressure chamber through the check valve. On the other hand, when a solenoid in the fuel discharge amount control valve is not energized in the fuel discharge stroke in which the plunger is caused to move upward from the lower end up to the upper end of the pump cylinder, the fuel discharge amount control valve is opened so that the fuel discharged from the high pressure fuel pump to the feed passage is returned to the inflow passage through the spill passage, as a result of which fuel is not fed to the fuel rail.
Moreover, when the solenoid in the fuel discharge amount control valve is constantly energized in the fuel discharge stroke, the fuel discharge amount control valve is closed whereby an amount of fuel corresponding to a maximum fuel discharge amount discharged from the high pressure fuel pump to the feed passage is fed to the fuel rail through the check valve. Also, when the solenoid is energized at a certain time during the fuel discharge stroke, the fuel discharge amount control valve is closed after the time point of the energization of the solenoid, so only the fuel discharged from the high pressure fuel pump to the feed passage during the upward movement of the plunger is fed to the fuel rail through the check valve.
As described above, by energizing the solenoid at a predetermined timing in the fuel discharge stroke, the amount of fuel to be discharged is adjusted to a desired amount within the range from zero to the maximum fuel discharge amount. Here, note that the energization start timing of the solenoid is uniquely determined from the amount of fuel to be discharged by storing a correlation characteristic between the energization start timing of the solenoid and the amount of fuel to be discharged in the ECU. In addition, in order to maintain the fuel pressure in the fuel rail at the present value, the flow rate of fuel injected by the fuel injection valve (fuel flowing out from the fuel rail) and the flow rate of fuel discharged by the high pressure fuel pump (fuel flowing into the fuel rail) need only to be controlled so that they become equal with each other.
Accordingly, in the above-mentioned first patent document or second patent document, a fuel discharge amount (an amount of fuel to be discharged) is determined by adding a fuel injection amount per cylinder injected from the fuel injection valve (feedforward amount: FF amount) to an amount of fuel to be discharged (feedback amount: FB amount) that is obtained based on a pressure deviation between a target fuel pressure set in accordance with the operating condition of the internal combustion engine and a fuel pressure detected by a fuel pressure sensor.
Here, the fuel injection amount is an amount that can be grasped by the ECU as a known amount of fuel flowing out from the fuel rail, so it is set as the FF amount to supplement the amount of outflow fuel. Also, the FB amount is a feedback correction amount that is calculated under proportional-plus-integral control or the like when a pressure deviation is resulted from accuracy variation or degradation of component parts or elements in the fuel feed system without regard to the FF amount being fed to the fuel rail.
However, the maximum fuel discharge amount in one discharge stroke of the high pressure fuel pump varies depending upon how many times fuel can be discharged to the fuel rail while a fuel injection stroke makes a round through the respective cylinders, i.e., during the time the internal combustion engine makes two revolutions (=720 degrees in crank angle (deg CA)). That is, the calculation timing of the fuel injection amount in the four-cylinder internal combustion engine includes a timing at which the fuel injection valve is actually driven, a first calculation timing to execute or actuate a fuel injection amount calculation section and a fuel injection valve control setting section, and a second calculation timing to execute or actuate an FF amount calculation section, an FB amount calculation section, and a fuel discharge amount control section. The relation between the fuel discharge stroke in which fuel is actually discharged and the fuel discharge amount (the amount of fuel to be discharged) becomes as follows.
For instance, in the case where the number of crests of the cam in the high pressure fuel pump is “2”, regarding the first calculation timing, four such timings are set at intervals of 180 deg CA within the range of 720 deg CA in which the fuel injection stroke to the respective cylinders makes a round. The amounts of fuel to be injected into the individual cylinders are respectively calculated, and predetermined injection timings and predetermined fuel injection pulse widths are respectively set.
On the other hand, regarding the second calculation timing, two such timings are set at intervals of 360 deg CA within the range of 720 deg CA in which the fuel injection stroke to the respective cylinders makes a round. In one of the second calculation timings, a total of the amounts of fuel to be injected for two cylinders is calculated as an FF amount and discharged in one discharge stroke. Also, in the other second calculation timing, a total of the amounts of fuel to be injected for the two remaining cylinders is calculated as an FF amount and discharged in another discharge stroke. As a result, the incoming and outgoing fuel balance of the fuel rail becomes zero as a whole and is thus maintained at the constant fuel pressure.
Thus, in case of the number of cam crests=2, the fuel discharge cycle is the same period (2 discharges for every two revolutions of the four-cylinder internal combustion engine) as the number of revolutions per minute of the engine, so the discharge cycle can be performed at a relatively low speed, and it is the most advantageous in terms of wear resistance in the sliding surfaces between the plunger and the cylinder of the high pressure fuel pump and in the contact surfaces between the cam and the plunger. However, a maximum fuel injection amount substantially at least twice as much as the maximum fuel discharge amount is needed, so there is a problem that the volume of the pressure chamber is enlarged, and the maximum fuel discharge amount, being substantially large, increases stress on the contact surfaces between the cam and the plunger thereby to deteriorate durability.
Next, explaining the case of the number of cam crests=3, regarding of the first calculation timing, the amounts of fuel to be injected into the respective cylinders are calculated at four first calculation timings, respectively, and predetermined injection timings and predetermined fuel injection pulse widths are set, as in the case of the number of cam crests=2.
On the other hand, regarding the second calculation timing, three such timings are set at intervals of 240 deg CA within the range of 720 deg CA in which the fuel injection stroke to the respective cylinders makes a round. In a first one of the second calculation timings, a total of the amounts of fuel to be injected for two cylinders is calculated as an FP amount and discharged in the discharge stroke. Also, in a second one of the second calculation timings, the amount of fuel to be injected for one of the two remaining cylinders is calculated as an FF amount, and the FF amount equal to the fuel injection amount is discharged in the discharge stroke. Further, in a third one of the second calculation timings, too, the amount of fuel to be injected for the other one of the two remaining cylinders is calculated as an FF amount, and the FF amount equal to the fuel injection amount is discharged in the discharge stroke. As a result, the incoming and outgoing fuel balance of the fuel rail becomes zero as a whole and is thus maintained at the constant fuel pressure.
Thus, in case of the number of cam crests=3, the fuel discharge cycle is a period 1.5 times as large as the number of revolutions per minute of the engine (3 discharges for every two revolutions of the four-cylinder internal combustion engine). Therefore, the discharge period becomes higher speed (i.e., shorter) than at the time of the number of cam crests=2, thus resulting in a disadvantage in terms of wear resistance in the sliding surfaces between the plunger and the pump cylinder and in the contact surfaces between the plunger and the cam, as compared with the case of the number of cam crests=2. Besides, since the maximum fuel discharge amount corresponding to twice the maximum fuel injection amount is needed, as in the case of the number of cam crests=2, there still remains the problem that the volume of the pressure chamber is enlarged, and stress on the contact surfaces between the cam and the plunger is increased to deteriorate durability.
Next, explaining the case of the number of cam crests=4, regarding the first calculation timing, the description is the same as that in the cases of the number of cam crests=2 and the number of cam crests=3. On the other hand, regarding the second calculation timing, four such timings are set at intervals of 180 deg CA within the range of 720 deg CA in which the fuel injection stroke to the respective cylinders makes a round, and the amount of fuel to be injected for one cylinder is calculated at each calculation timing as an FF amount, and the FF amount equal to the fuel injection amount is discharged in each discharge stroke. As a result, the incoming and outgoing fuel balance of the fuel rail becomes zero as a whole and is thus maintained at the constant fuel pressure.
As described above, in case of the number of cam crests=4, the maximum fuel discharge amount can be made equal to the maximum fuel injection amount, so the volume of the pressure chamber can be reduced to a minimum, and the actual maximum fuel discharge amount can also be small, as a result of which stress on the contact surfaces between the plunger and the cam is reduced, and it is the most advantageous in terms of durability. However, since the fuel discharge cycle is a period equal to twice the number of revolutions per minute of the engine (i.e., four discharges for every two revolutions of the four-cylinder internal combustion engine), the discharge cycle becomes the highest speed, thus causing a problem that wear resistance in the sliding surfaces between the plunger and the pump cylinder and in the contact surfaces between the plunger and the cam is deteriorated.
Thus, in order to improve the reliability of the high pressure fuel pump, it is desirable to increase the fuel discharge period and reduce the maximum fuel discharge amount, but there arises a problem that in case where such a scheme is intended to be applied to a high power internal combustion engine, it becomes difficult to provide durability and wear resistance of the high pressure fuel pump, thereby making it impossible to ensure reliability.
That is, when comparing the characteristic of the high pressure fuel pump with respect to the number of cam crests, in the relation between the discharge cycle and the maximum fuel discharge amount of the high pressure fuel pump, it is desired that the discharge cycle be set to be at as low speed as possible and the maximum fuel discharge amount be set to be as small as possible. However, it is difficult to improve reliability more than the present state without any great increase in cost. In particular, there is a problem that in case of high power four-cylinder internal combustion engines in which the maximum fuel injection amount is further increased, it is impossible to make durability and wear resistance of the high pressure fuel pump compatible with each other.
Here, it is considered, for instance, that by installing two high pressure fuel pumps with two cam crests and driving them in parallel, the discharge period is made to be at low speed and at the same time the volume of the pressure chamber is reduced, thereby making durability and wear resistance compatible with each other. In this case, however, a large increase in cost will be induced, and it can not be said a practical method of solution at all even from the viewpoint of installation. In the known fuel pressure control apparatuses for a multicylinder internal combustion engine, in case of the number of cam crests of the high pressure fuel pumps being equal to 2, the maximum fuel discharge amount corresponding to twice the maximum fuel injection amount is required and hence there has been the problem that the volume of the pressure chamber is enlarged, and the maximum fuel discharge amount, being substantially large, increases stress on the contact surfaces between the cam and the plunger thereby to deteriorate durability. In addition, in case of the number of cam crests=3 or 4, the discharge period becomes high speed (i.e., short), resulting in a disadvantage in terms of wear resistance on the sliding surfaces between the plunger and the pump cylinder and on the contact surfaces between the plunger and the cam. Besides, there is also the problem that the volume of the pressure chamber is enlarged, and stress on the contact surfaces between the cam and the plunger is increased to deteriorate durability, as in the case of the number of cam crests=2.